Considerable attention has been directed to providing improved corrosion protection to metallic surfaces. One way of providing this corrosion protection is by electrodepositing a zinc coating on the surface. For decades, electroplated zinc has been used by the automative industry to provide an economical, highly corrosion-resistant coating. However, with today's unprecedented demands for higher quality and extended warranties, both the automotive manufacturers and their suppliers have had to develop new coatings. Zinc deposits from electroplating baths have been modified to provide improved overall brightness, range of allowable current densities and ductility. Good overall performance also is being demonstrated by zinc-cobalt and zinc-nickel alloy platings. These alloys are being introduced as replacements for conventional zinc electroplates in automotive as well as other applications requiring extended corrosion-resistance. The term "alloy", as used in this specification and claims is defined as a mixture of two or more metallic elements which may be microscopically homogeneous or microscopically heterogeneous.
In recent years, the activity in the plating area has been concentrated on the development of cyanide-free alkaline baths or improvements in acid plating baths. This invention relates to acid zinc and zinc-alloy baths. Typically, acid zinc plating baths are based on a suitable inorganic zinc salt such as zinc chloride or zinc sulfate, and the baths usually include buffers such as the corresponding ammonium salt. Other additives are included in the baths to promote and improve ductility, brightness, throwing power and covering power. Surface active agents are normally included to improve crystal structure, reduce pitting, and increase the solubility of other additives.
Brighteners for acid zinc baths are described in U.S. Pat. No. 3,920,528 which are compounds having a carboxyl or a sulfo group attached directly or through an alkylene, vinyl, carbonyl or phenyl group to various heterocyclic rings including pyrrolidine or pyrrolidone.
Polyvinylpyrrolidone (PVP) has been incorporated into zinc plating baths to impart various desirable properties. U.S. Pat. No. 4,397,717 describes alkaline zinc electroplating baths which contain various additives including polyvinylpyrrolidone. The use of polyvinylpyrrolidone in acid zinc plating baths is described in U.S. Pat. Nos. 3,594,291, 3,891,520, 3,919,056, 4,226,682 and 4,444,630. In U.S. Pat. No. 3,594,291, a brightener system is described for acid zinc plating solutions which comprises the combination of N-polyvinylpyrrolidone with a secondary brightener which is a carbonyl compound such as acetophenone, benzalacetone, etc. The concentration of the polyvinylpyrrolidone in the bath is between 0.5 to 100 g/l. The combination is reported to produce a level of brightness which is greater than the individual effects. Polyvinylpyrrolidone is described as a surfactant in the acid zinc plating baths described in U.S. Pat. No. 3,919,056, and U.S. Pat. No. 4,226,682 describes the use of certain specified dispersing agents including PVP in combination with a brightener compound such as 1-phenyl-1-penten-3-one.
U.S. Pat. No. 2,495,668 describes plating baths for electrodepositing copper wherein the plating bath contains, in addition to copper cyanide, sulfur compounds such as .beta.-mercaptoethanol, dithiodiglycol, .beta.,.beta.'dihydroxyethyl sulfide and thioglycolic acid. Polymeric sulfur-containing compounds having the general formulae RS(R'O).sub.n H and S[(R'O).sub.n H].sub.2 wherein R is an alkyl group containing up to about 24 carbon atoms, each R' is independently an alkylene group containing 2 or 3 carbon atoms, and each n is independently an integer of from 1 to about 100 have been described in U.S. Pat. No. 4,229,268 as being useful in providing a level and bright zinc deposit from acid zinc plating baths. In U.S. Pat. No. 4,832,802, these polymeric sulfur-containing compounds, in combination with a ductility-improving amount of at least one specified acetylenic derivative, provide an additive composition for aqueous acidic zinc-nickel plating baths useful in depositing level and bright zinc-nickel alloy deposits.